1. Field of the Invention
The present invention relates to an optical system for a liquid crystal projector and more particularly, for a liquid crystal projector having three liquid crystal display panels corresponding to the three primary colors.
2. Description of Related Art
A liquid crystal projector is an appratus for projecting a color optical image corresponding to a provided image information signal onto a screen. The liquid crystal projector has an optical system therein for forming the color optical image.
In FIG. 3, an optical system of a liquid crystal projector is comprised of a light source 101, a curved reflecting mirror 102, a UV/IR (ultraviolet rays/infrared rays) cut filter 103, a red-light-reflecting color separation dichroic mirror 104, a green-light-reflecting color separation dichroic mirror 105, reflecting mirrors 106, 107 and 108, relay lenses 109 and 110, condenser lenses 111, 112 and 113, polarizers 114, 115 and 116, a red-light modulating LCD (liquid crystal display) panel 117, a green-light modulating LCD panel 118, a blue-light modulating LCD panel 119, a red filter 120, a cross dichroic prism 121 and a projection lens 122.
A white beam emitted from the light source 101 is reflected by the curved reflecting mirror 102, and travels as a substantially parallel beam. This parallel beam is separated into red, green and blue light by the red-reflecting color separation dichroic mirror 104 and the green-reflecting color separation dichroic mirror 105.
The red light comes into the red-light modulating LCD panel 117 through the reflecting mirror 106, the condenser lens 111, and the polarizer 114.
The green light comes into the green-light modulating LCD panel 118 through the condenser lens 112 and the polarizer 115.
The blue light comes into the blue-light modulating LCD panel 119 through the relay lens 109, the reflecting mirror 107, the relay lens 110, the reflecting mirror 108, the condenser lens 113, and the polarizer 116.
The red-light modulating LCD panel 117 optically modulates the incident red light and generates as an outgoing red optical image corresponding to the red component image information signal.
The green-light modulating LCD panel 118 optically modulates the incident green light and generates an outgoing green optical image corresponding to the green component image information signal.
The blue-light modulating LCD panel 119 optically modulates the incident blue light and generates an outgoing blue optical image corresponding to the blue component image information signal.
These optical images from each LCD panel 117, 118 and 119 are synthesized in cross dichroic prism 121 and comes into the projection lens 122.
The projection lens 122 enlarges the synthesized images and projects them onto a screen.
However, the light emitted from the green-light modulating LCD panel 118 has a certain spread as shown in FIG. 4, and the cross dichroic prism 121 shows incident angle dependency as shown in FIG. 5, such that the light having a large incident angle shifts to shorter wavelengths, while the light having a small incident angle shifts to longer wavelengths, so that the light having a large incident angle to a red reflecting plane is reflected by a red reflecting plane. The green light emitted from the green-light modulating LCD panel 118 is reflected by a red reflecting plane of the cross dichroic prism 121, and goes to the back surface of the blue-light modulating LCD panel 119. Therefore the contrast of the blue-light modulating LCD panel 119 is deteriorated.
Moreover if an absorption type blue-light penetration filter is disposed to prevent the green light from coming into the back surface of the blue-light modulating LCD panel 119, the white balance is deteriorated because only the absorption type blue-light penetration filter with a low penetration is available at present.
Furthermore, if only an reflection type blue-light penetration filter is disposed to prevent the green light from coming into the back surface of the blue-light modulating LCD panel 119, the green light reflected by the reflection type blue-light penetration filter is reflected by the red reflecting plane of the cross dichroic prism 121 and goes to the back surface of the green-light modulating LCD panel 118. Therefore the contrast of the green-light modulating LCD panel 118 is deteriorated.